Origen

We think of rules as giving us lines to follow, and the word goes back to Latin regula ‘straight stick’, and beyond that to regere ‘to rule’, the source of regency and royal (Late Middle English). To rule the roost is to be in complete control. The original form of the phrase was rule the roast, from the end of the 15th century, which may imply that it referred to the most important person at a banquet or feast. Roast changed to roost in the 18th century when people started thinking about a cockerel asserting itself over the other roosting birds in the farmyard. The rule in run the rule over, ‘to examine quickly’, is a measuring stick or ruler. It has the same meaning in rule of thumb, ‘a broadly accurate guide based on practice rather than theory’. This expression, recorded from 1692, is probably from the ancient use of parts of the body, such as the foot and the hand, as units of measurement. The first joint of a man's thumb is about an inch long, and so is useful for making rough measurements when you have mislaid your ruler. See also rail